Sheet Metal Stamping Simulation

The inside radius of a bent section.

Source: The OHIO State University

Bend radius, which is measured to the inside curvature, is the minimum radius one can bend a pipe, tube, sheet, cable or hose without kinking it, damaging it, or shortening its life. The smaller the bend radius, the greater is the material flexibility (as the radius of curvature decreases, the curvature increases). The diagram below illustrates a cable with a seven-centimeter bend radius. The minimum bend radius is the radius below which an object such as a cable should not be bent.

Source: Wikipedia

The radius corresponding to the curvature of a bent specimen or part, as measured at the inside surface of the bend.

Source: Answers.com